The present invention relates to a cylindrical printing blanket that has high durability and high productivity and is particularly suited to high-speed offset printing.
Various types of printing blanket have been developed and used. One example is a sheet-like blanket made by laminating a base fabric layer, a compressive layer made of a porous oil-resistant rubber, a top layer fabric and NBR (acrylonitrile-butadiene copolymer rubber) oil-resistant rubber. This sheet-like blanket is set on a cylinder by fitting tabs attached on both ends of the blanket into gaps formed in the printer cylinder, pulling the tabs on both ends by means of a take-up mechanism that is incorporated in the cylinder thereby applying a tension to the blanket.
However, the sheet-like blanket has such problems as 1) vibration occurs when the gap passes through a nip during rotation of the printer cylinder, resulting in such defects of the printing quality as shock-streak and making high-speed printing at 1000 rpm or higher impossible; 2) it takes as long as 30 minutes to exchange the blanket leading to a low efficiency of changing operation; and 3) setting the blanket on the cylinder while applying tension thereto tends to cause set in fatigue and thinning of the blanket due to the tension that is constantly applied thereto.
To avoid the problems, a cylindrical printing blanket having no seam in the circumferential direction was developed. This printing blanket has such a general structure that a compressive layer made of a porous oil-resistant rubber which has no seam, a non-elastic layer and a surface printing layer made of NBR oil-resistant rubber which has no seam are laminated in this order, via a seamless adhesive layer made of an elastomer, on the outer surface of a cylindrical sleeve that is put on a blanket cylinder from the outside (Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) 5-301483). The cylindrical blanket is used while being put on a printer cylinder that does not have gaps. The sleeve has an inner diameter that is equal to or slightly smaller than the outer diameter of the cylinder and is normally engaged firmly with the cylinder, but expands slightly in the radial direction when a pressure is applied from the inside and can be removed from the cylinder. The cylinder has air holes formed therein to supply a pressured gas onto the inner surface of the sleeve.
The cylindrical blanket has such advantages over the sheet-like blanket as: 1) structure of the printer cylinder without gap does not generate vibration during rotation and does not cause shock-streak on the printed matter, thereby making high-speed printing possible and achieving high productivity of printing; and 2) the blanket can be exchanged in a short time of one minute and does not require the adjustment of tension unlike the sheet-like blanket, thus allowing easy changing operation without requiring skill.
Use of the sheet-like blanket of the prior art in printing involves the problems 1) to 3) as described previously. The cylindrical blanket was developed for the purpose of solving the problems, but has the following problems left to be solved.
1) Influence on the Printing Quality
Although the shock-streak of the sheet-like blanket was solved by the use of the sleeve, the cylindrical blanket of the prior art has a thread layer made by winding a thread spirally around the circumference. The thread layer, also called the bearing layer, has the function of preventing a surface printing rubber layer and the compressive layer from experiencing shear deformation occurring in the direction of rotation during printing. The bearing layer has previously been called the stabilizer layer and has been recognized only to have the function described above. While the thread layer is a counterpart of the fabric layer in the case of the sheet-like blanket, the thread layer of the cylindrical blanket tends to cause a printing defect of an unintended density pattern being generated on the printed matter along the thread, due to winding of the thread around the circumference because the blanket is formed in a cylindrical shape. This is because the restricting force of the thread layer is weaker between adjacent lines of the thread, thus causing a difference in the shear deformation of the surface rubber that results in the unevenness in printing. When the surface printing rubber layer formed over the thread layer is made thicker for the purpose of preventing the problem described above, printing defects such as bulge and slur tend to occur.
2) Set in Fatigue of Compressive Layer
Set in fatigue due to tension as in the case of the sheet-like blanket does not occur in the blanket made in cylindrical shape with the different mounting mechanism. In the case of the cylindrical blanket, however, since the compressive rubber layer is always compressed by the thread layer formed by winding the thread with a tension around the compressive rubber layer, set in fatigue is likely to occur in the compressive layer.
3) Productivity of Manufacturing the Blanket
The sleeve-like blanket is difficult to manufacture in a long sheet of large width. Thus it is necessary to make in a plurality of laminated structure (a base layer, a compressive layer, a thread layer and a surface rubber layer) for each product, thus resulting in a low productivity.
An object of the present invention is to solve the problems of the cylindrical blanket of the prior art and to provide a cylindrical printing blanket that is capable of producing printed matters of high quality over a wide range from ordinary printing to high-speed printing, robust and easy to handle, has a long service life and allows easy reuse of the sleeve and other components.
In order to solve the problems described above, the present inventors have studied intensively about the structure of the cylindrical blanket, thus completing the present invention.
The present invention thus relates to the following printing blankets.
1) A cylindrical printing blanket comprising a seamless sleeve and a sheet-like blanket having a fabric layer, a compressive layer and a surface printing layer, said sheet-like blanket being bonded onto the outer surface of said seamless sleeve.
2) The cylindrical printing blanket according to the term 1) wherein said sheet-like blanket is bonded via a thread layer formed by winding a thread in spiral on a sleeve via an adhesive elastomer layer.
3) The cylindrical printing blanket according to the term 2), wherein a cylinder of diameter that is 0.05% to 1.0% smaller than that of a cylinder of a printing press whereon said cylindrical printing blanket is mounted is provided with a sleeve of a diameter equal to or slightly smaller than that of said cylinder being temporarily mounted thereon, and a thread is wound in spiral thereby providing an interference.
4) The cylindrical printing blanket according to the term 2), wherein a seam of said sheet-like blanket on the sleeve is processed to prevent a liquid from permeating at least through the end faces thereof.
5) The cylindrical printing blanket according to the term 4), wherein processing of the seam is to cover with a coat that does not allow liquid to permeate.
6) The cylindrical printing blanket according to the term 2), wherein a groove generated when said sheet-like blanket is bonded is filled with an elastomer.
7) The cylindrical printing blanket according to the term 6), which is filled with compressive elastomer.
The printing blanket of the present invention has the sheet-like blanket that comprises at least one fabric layer and a surface printing rubber layer is adhered onto the outer surface of the sleeve, while the seam is processed to prevent a liquid from penetrating therethrough.
In other words, the thread layer of the cylindrical printing blanket of the prior art, that is formed by winding the thread while applying a tension in a spiral as a bearing layer or stabilizer layer for the surface printing layer, is replaced by the fabric layer in the present invention. And the thread layer formed around the sleeve makes it easier to hold the sleeve on the cylinder in close contact therewith and also improves the durability of the sleeve. When the thread layer is provided directly on the sleeve via an adhesive layer, it is made easier to process and handle such as adhesion of the sheet-like blanket.
In contrast to the cylindrical printing blanket of the prior art that is based on the concept of giving the functions of holding onto the cylinder and durability against cracks and other defects to the sleeve itself, the present invention has been completed upon finding of important functions of the thread layer.
The cylindrical printing blanket of the present invention has the following effects.
1) Better Printing Quality
Since the blanket is formed in a cylindrical shape, shock-streak will not be generated. The blanket of the present invention is mounted on a printer cylinder by aligning the seam of the printing plate and the seam of the blanket. Unlike the prior art, woven or non-woven fabric is used, instead of the thread layer, directly below the surface printing rubber layer, and a plain weave fabric layer is generally formed. As a result, unevenness in printing due to the surface unevenness of the threads is not produced.
2) Mitigation of Set in Fatigue
Since compressive force is not applied by the thread layer to the compressive layer unlike the cylindrical printing blanket of the prior art, less set in fatigue is caused. Also because tension is not constantly applied when mounted unlike the sheet-like blanket of the prior art, this also contributes to the mitigation of the set in fatigue.
3) Higher Productivity of Manufacturing
The printing blanket of the present invention can be manufactured in a long sheet of large width in a separate production line in advance, cutting the sheet to a proper size and bonding the cut sheet onto the sleeve that serves as a support. Thus the blanket can be manufactured with a very high production efficiency.
4) High Productivity of Printing
In the prior art, it has not been considered possible to use such a cylindrical blanket of a simple structure as that of the present invention in high-speed printing. However, it was found that higher durability can be achieved by using such a structure as the sleeve is held by the thread layer. Even when the amount of printing per unit blanket is a little less than in the case of the conventional blanket, high productivity of manufacturing the blanket makes it possible to increase the amount of printing that can be made per unit price.